1. Field of the Invention
The present invention relates generally to an apparatus and method for use in marine seismic exploration and more particularly to an energy source for generating pressure pulses in a body of water.
2. Description of the Prior Art
In marine seismic exploration, to obtain geophysical information relating to the substrata located below a body of water, acoustic energy in the form of pressure pulses or shock waves is released into the water every few seconds. The shock waves propagate into the substrata beneath the water from where they are reflected and refracted back to the water. The returning shock waves are detected by a plurality of sensors (usually hydrophones) placed in seismic cables towed behind a vessel. The useful data contained in the signals produced by the sensors is processed to determine the geophysical structure of the surveyed substrata.
Air guns or gas guns are most commonly used as the acoustic energy sources or transmitters to produce shock waves having known characteristics. State of the art air guns normally have an annular housing which contains an integral chamber for storing a volume of compressed air. A sleeve-type shuttle sealingly encloses a portion of the housing to prevent compressed air from escaping into the surrounding medium, which is usually water. Means are provided to move the shuttle to break the seal, which permits the compressed air to escape into the surrounding water. The air guns which use a sleeve-type shuttle as described above are sometimes referred to in the oil and gas industry as the sleeve-type air guns. U.S. Pat. No. 4,472,794, issued to Chelminski on Sep. 18, 1984, U.S. Pat. No. 4,623,033 (the "'033 Patent"), issued to Harrison on Nov. 18, 1988 and U.S. Pat. No. 5,001,679 (the "'679 Patent") also issued to Harrison on Mar. 19, 1991, disclose two of the more commonly used sleeve-type air guns for performing marine geophysical surveying.
Chelminski discloses an air gun that has a housing which defines a main chamber for storing compressed air and a firing chamber. The main chamber has a discharge port. A movable sleeve type shuttle is placed within the housing to sealingly enclose the discharge port when it is in its normal closed position. A solenoid valve applies high pressure air to the firing chamber, which causes the sleeve shuttle to move away from the closed position. A relatively long acceleration distance for the sleeve shuttle is provided so that the sleeve shuttle may attain a high velocity before it passes over the discharge ports. When the sleeve shuttle opens the ports, the compressed air discharges from the main chamber to the surrounding medium creating a pressure pulse.
The Harrison air gun disclosed in the '033 patent also contains a housing that has a main chamber for storing compressed air. The housing has an annular opening around the main chamber. A sleeve is affixed around the housing that encloses a substantial portion of the main chamber. One face of the fixed sleeve contains a groove for housing a seal. A single sleeve-type shuttle having a surface adapted to abut against the face of the fixed sleeve containing the seal is slideably placed around the housing. The shuttle is pressed against the fixed sleeve to sealingly enclose the pressurized air in the main chamber. To discharge the pressurized air into the surrounding medium, the shuttle is moved away from the sleeve thereby allowing the compressed air to escape into the surrounding medium. After the pressurized air has escaped, the shuttle is forced against the fixed sleeve and the main chamber is filled with the compressed air for the next firing cycle.
The Harrison air gun disclosed in the '033 patent and the Chelminski air guns have been found to take several milliseconds to release most of the compressed air from their respective main chambers. An ideal pressure pulse will be obtained when the entire air can be released instantaneously. On a pressure versus time graph, such an ideal pulse will be a spike at the moment the air is released.
The second Harrison air gun disclosed in the '679 patent is a modified version of the '033 air gun. The '679 air gun, like the '033 air gun, contains a fixed sleeve having a fixed seal that encloses substantially all of the main chamber and a shuttle which abuts against the fixed sleeve to seal the chamber. A second shuttle 24 containing a lip is placed around a portion of the final sleeve. To fire the air gun, the first shuttle 18 is moved away from the fixed sleeve 12, which allows the pressurized air to pass through the opening created between the fixed sleeve and the shuttle. This air then forces only the second shuttle but not the seal to move a relatively short distance away from the first shuttle.
Both the '033 and the '679 air guns have a shuttle which abuts against a fixed sleeve. The fixed sleeve enclosed most of the chamber. The size of the opening for discharging the compressed air is defined by the movement of the shuttle. The opening is made near the end of the fixed shuttle, thereby forcing the compressed air to travel nearly the entire length of the chamber before it can discharge into the surrounding medium. Further, the '679 air gun is much more complex to manufacture than the '033 air gun, without offering any significant advantages over the '033 air gun.
The quality of the pressure pulses produced by the Harrison and Chelminski air guns is compromised due to the relatively low speed with which the opening is created and by placing the opening near the end of the main chamber by the size of the opening created for discharging the pressurized air in a given period of time. The pulse shape will be improved if a larger escape opening can be created at the geometric center of the main chamber in the same or less time period as compared to these prior art air guns. It is, therefore, highly desirable to have an air gun which discharges the pressurized air in less time than the presently used air guns. The present invention provides an air gun which is capable of discharging substantially all of the pressurized air in less time as compared to the presently used air guns.
In practice, several different capacity air guns placed in a spaced relationship are used to produce a pressure wave signature to perform geophysical surveying. The spaced arrangement is referred to as an array or subarray. One or more than one air gun arrays are towed beneath the water surface behind a marine vessel. The capacity of an air gun may be changed by installing inserts therein to alter the air chamber effective volume. In this manner, the same air gun may be used to cover a given capacity range. For example, an air gun having a maximum capacity of 300 cubic inches may be used to cover a range up to the maximum capacity of 300 cubic inches. Because of the great weight and overall size of the air guns, it is highly desirable to have an air gun which is smaller in size but has larger capacity. Such an air gun, apart from being lighter, may be used for a greater variety of applications. The above noted prior art air guns are, by their design configuration, limited in their capacity. The present invention provides an air gun which has a much greater capacity compared to a comparable size prior art air gun.
The cost to perform marine geophysical surveying is extremely high. It requires the use of, among other things, a vessel, electronic processing equipment, computers, marine sensor cables and highly skilled personnel). The equipment costs several million dollars and the total per hour operating cost is several thousand dollars. Due to the high cost, the surveying activity is performed essentially around the clock for several days or weeks at a time. Since air guns are fired every few seconds for extended periods of time, they frequently fail. This usually requires stopping the entire operation in order to repair or replace the failed air guns. It is, therefore, desirable to have a very reliable air gun which has fewer components and which can be assembled, disassembled and repaired in less time than the prior art air guns. The prior art air guns are fairly complex in their structure and contain a large number of parts. The air gun of the present invention contains less number of parts and is easier to assemble and disassemble compared to the prior art air guns noted above.